The new Third Generation Partnership Project (3GPP) standard known as Long Term Evolution (LTE) (3GPP LTE Rel-10) supports heterogeneous networks. In heterogeneous networks, a mixture of cells of different size and overlapping coverage areas are deployed. For example, a heterogeneous network may deploy pico cells served by relatively low power nodes within the coverage area of a macro cell served by relatively high power base stations. Heterogeneous networks could also deploy relatively low-power home base stations and relays to provide improved service in indoor areas. The aim of deploying low power nodes, such as pico base stations, home base stations, and relays, within a macro cell where coverage is provided by a high power base station, is to improve system capacity by means of cell splitting gains as well as to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployment schemes represent one alternative to deployment of denser networks of macro cells and are particularly effective to cover traffic hotspots, i.e., small geographical areas with high user densities served by lower power nodes.
In heterogeneous networks, there may be a large disparity in output power of the low power nodes compared to the base stations serving macro cells. For example, the output power of the base stations in the macro cells may be in the order of 46 dBm, while the output power of the low power nodes in the pica cells may be less than 30 dBm. In some heterogeneous networks, the macro cells and pico cells operate on the same carrier frequencies and inter-cell interference coordination (ICIC) techniques are used to deal with interference when user terminals are operating in areas served by both macro cells and pico cells. For example, the transmission of the primary synchronization signals (PSS) and secondary synchronization signal (SSS) by a pico cell can be offset in time relative to the PSS and SSS transmitted by the macro cell. The macro cell can then avoid scheduling downlink transmissions in subframes when the pico cell is transmitting the PSS and SSS. Similarly, the pica cell can avoid scheduling downlink transmissions on resource elements that are used by the macro cell for transmitting the PSS and SSS.
The time-shifting of radio frames is not without drawbacks. In Time Division Duplex (TDD) systems where downlink (DL) and uplink (UL) transmissions occur on the same carrier but at different times, time shifting of radio frames across the macro and pico layers to resolve collisions of PSS/SSS would mean that a DL transmission on one layer collides with an UL transmission on the other layer. Time shifting may also be less attractive in Frequency Division Duplex (FDD) systems if the carrier supports MBSFN transmission because radio frame alignment can simplify the overall configuration.
Therefore, there is a need for ICIC techniques that do not require time-shifting of radio frames.